Normal
This happens from time to time - contact the Graduate Recruitment team - it is unlikely to be an issue. The worst that will happen is the firm won't be able to provide you with an alternative date for the assessment centre but that is no different a position than you are in now. They may, however, be able to move you to an alternative AC date or find alternative options - but they will only be able to offer that if you speak to them. I would try and call them tomorrow (call them in the first instance rather than rely on email).
